1. Field of the Invention
The present invention relates to a water-cooled internal combustion engine cooled by circulating cooling water. More specifically, the invention relates to a structure forming water jackets in the cylinder head of a water-cooled internal combustion engine to be applied to, for example, an outboard motor.
2. Description of the Related Art
The cylinder head of a known water-cooled internal combustion engine disclosed in, for example, JP-A 2000-159190 is provided with an exhaust manifold passage through which the exhaust gas discharged from a plurality of combustion chambers flows, and water jackets including a combustion chamber water jacket surrounding combustion chambers and an exhaust passage water jacket surrounding the exhaust manifold passage.
When the exhaust manifold passage is formed in the cylinder head, it is preferable, in view of improving the durability of the cylinder head, to reduce differences in temperature among combustion chamber walls forming the combustion chambers and exhaust passage walls forming the exhaust passages including the exhaust manifold passage, i.e., to make the temperatures of the combustion chamber walls and the exhaust passage walls uniform. The combustion chamber water jacket has an intricate arrangement of passages because the cylinder head is provided with intake valves, exhaust valves and ignition plugs. Thus the cooling water has difficulty in smoothly flowing through the combustion chamber water jacket as compared with flowing through the exhaust passage water jacket having a comparatively simple arrangement of passages. Therefore, if the combustion chamber water jacket and the exhaust passage water jacket are connected simply, the respective temperatures of the combustion chamber walls and the exhaust passage walls are likely to differ from each other and the uniformity of temperature distribution in the cylinder head is worsened.
Generally, the exhaust passage water jacket and the exhaust passages including the exhaust manifold passage are formed, for example, by cores placed in a master mold for casting the cylinder head. Although it is preferable to surround a large part of the exhaust manifold passage by a water jacket to cool the exhaust passage walls forming the exhaust manifold passage efficiently, a casting mold including cores having an intricate shape is needed to form such a water jacket surrounding cores for forming the exhaust manifold passage. The mold and cores having such an intricate shape increases the manufacturing cost of the cylinder head. When only the exhaust passages are formed by using cores, not to mention when both the water jackets and the exhaust passages are formed by using cores, it is desirable that positions of core supports for supporting the cores in the master mold do not make the mold for forming the water jackets intricate and facilitate supporting the cores in the master mold. When a through hole for receiving an exhaust gas measuring device is extended through the water jacket, the area of parts of the exhaust passage walls covered with the water jacket decreases and the exhaust passage wall cooling effect is reduced accordingly.
The present invention has been made in view of the foregoing problems and it is therefore an object of the present invention to improve the uniformity of temperature distribution on the combustion chamber walls and the exhaust passage walls of a water-cooled internal combustion engine provided with an exhaust manifold passage to improve the uniformity of temperature distribution in the cylinder head of the water-cooled internal combustion engine.
Another object of the present invention is to provide a water-cooled internal combustion engine provided with an exhaust passage water jacket having a simple shape, surrounding an exhaust manifold passage formed in the cylinder head, and capable of exercising a necessary cooling effect and of facilitating supporting cores for forming an exhaust manifold passage. A further object of the present invention is to facilitate placing a core for forming an exhaust manifold passage in a mold, to improve the stability of the core for forming the exhaust manifold passage, to facilitate parting molds and to reduce the cost of a cylinder head by avoiding increasing through holes opening into the exhaust manifold passage.